1. Field of the Invention
This invention relates to a method of purifying hydroxphenylalkanes, including tri(hydroxyphenyl)alkanes, such as 1,1,1-tris(4'-hydroxyphenyl) ethane (hereinafter identified for brevity as "THPE"), and more particularly, to methods which employ alkali metal dithionites in connection with such purification.
2. Description of Prior Art
THPE may be produced by reacting 4-hydroxyacetophenone with phenol. Typically, phenol is also used as the solvent. The reaction proceeds under acidic catalytic conditions, such as with a co-catalyst system of hydrochloric acid and beta-mercaptopropionic acid. The product contains substantial amounts of impurities which include mixtures of ortho and para tris(hydroxyphenol)ethane isomers, 1,1-bis(hydroxyphenyl)ethylene isomers, phenol, 4-hydroxyacetophenone, chlorides, and unidentified colored bodies and light and heavy ends. This crude reaction product is reddish-brown. A major application of THPE is use as a polycarbonate chainbranching agent. Color, transparency, and compound purity are extremely important in this application. Accordingly, the reaction product must by purified to isolate the THPE, and the THPE isolate must have low color and high transparency.
In U.S. Pat. No. 4,992,598, issued Feb. 12, 1991, assigned to the assignee of this invention, a process is described for purifying THPE resulting from the reaction of 4-hydroxyacetophenone with phenol. The process comprises:
(a) washing a crude product mixture of THPE and impurities resulting from the reaction, with a saturated solution of THPE in a solvent comprising from about 60% to 70% by weight of water and from about 25% to about 40% by weight of methanol; PA0 (b) isolating the washed crude product mixture from the formed effluent washing composition, and dissolving the washed crude product mixture in methanol; PA0 (c) optionally adding first sodium borohydride and then activated carbon to the washed crude mixture dissolved in methanol, and then filtering off the carbon; PA0 (d) adding sufficient dilute aqueous sodium borohydride to the dissolved, washed crude mixture to form a precipitate of THPE; PA0 (e) filtering the precipitate to form a purified THPE and filtrate; PA0 (f) washing the filtered THPE precipitate with an aqueous methanol solution, which optionally contains THPE to saturation, and conducting the rinsing for a sufficient time to remove substantially all residual colored impurities from the precipitate; and PA0 (g) optionally rinsing the filtered precipitate with an aqueous solution of sodium dithionite.
Sodium dithionite is widely used as a bleaching agent. Oxygen should be excluded during all steps because aqueous S.sub.2 O.sub.4.sup.2- is easily oxidized. Sodium borohydride is used extensively for the reduction of organic compounds. It has the ability to reduce aldehydes and ketones selectively in the presence of other functional groups, and, also, is useful in reducing carbonyl unsaturation, which can give rise to color. Notwithstanding use of sodium borohydride or sodium dithionite to advantageously reduce color in crude THPE cake according to the method of the above-described process, deeper color reduction is desirable.